Abstract
Captures of northern pike on the spawning grounds and measures of egg deposition showed that from 1975 to 1978 spawning occurred in April, after the onset of spring high waters and the warming of the water above 5–6 °C. Maximal egg deposition occurs on abandoned meadows, pastures, and shrub–grass areas, in water 60 cm or less. In the Bleury Stream area, these substrates lie mostly between elevations 30 and 30,5 m. Water temperature influences the length of the spawning and incubation periods. Spawning, egg incubation, and growth of the young to a mean length of 20 mm require approximately 40 days. From 1970 to 1977, two strong year classes (1973 and 1976) were produced; they were both followed by a weak year class. Air temperatures in June, summer water levels (June–September), and also the strength of the preceding year class, had a determining effect on year class strength.
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